"The Passage of Power," the fourth volume in Robert Caro's epic biography of Lyndon Baines Johnson, encompasses the period of LBJ's deepest humiliation and his greatest accomplishment. It is a searing account of ambition derailed by personal demons in Johnson's unsuccessful bid for the 1960 Democratic presidential nomination. It is a painful depiction of "greatness comically humbled" when Johnson gave up his unbridled authority as Senate majority leader to becomeJohn F. Kennedy's disdained vice president. Most of all, it is a triumphant drama of "political genius in action" as Johnson smoothly took up the reins of office in the wake of Kennedy's assassination, made his predecessor's liberal agenda his own, and bent Congress to his will as Kennedy had never been able to do. Caro combines the skills of a historian, an investigative reporter and a novelist in this searching study of the transformative effect of power — its possession, its loss, its restoration — on the character and destiny of a man who from his teens had one overriding goal: to be president of the United States.

With each volume (a fifth is promised on Johnson's final decade), the biography gains a cumulative richness that mostly justifies its length and Caro's fondness for emphatic repetition, as well as frequent digressions into wonderful background material. The childhood roots of Johnson's all-consuming political aspirations, cogently traced in Volume 1, "The Path to Power," explain his otherwise mystifying indecision about openly pursuing the presidency until it was much too late in 1960: He still feared the kind of public failure that had shamed his father. Johnson's willing participation in political corruption and shady financial dealings, scathingly portrayed in Volume 2, "Means of Ascent," threatened Johnson's prospects in 1963, when a major scandal involving his protégé Bobby Baker seemed likely to get him dropped from the 1964 Democratic ticket. The unerring grasp of arcane congressional rules and the formidable gift for personal manipulation displayed by Majority Leader Johnson in Volume 3, "Master of the Senate" (probably the best book ever written about the American legislative process), enabled President Johnson to overcome a 57-day filibuster and force passage of the landmark Civil Rights Act of 1964.

An occasional difficulty in the previous volumes, and a more severe problem in Caro's biography of Robert Moses, "The Power Broker," is that the author's acid treatment of his subjects' often despicable actions sometimes prompts readers to wonder why he is writing about these contemptible characters. The answer, of course, is that Moses and Johnson were powerful men who had an enormous effect on our urban and national development and that understanding the way power is used and abused is vital to preserving a meaningful democracy. Still, there have been times when, faced with the revelation of yet another dirty deal or youthful ideal betrayed, reading Caro's work felt — in those volumes — like assignments for a particularly depressing political science class.

Not so with "The Passage of Power." Caro evinces genuine empathy for Vice President Johnson, mockingly dubbed "Rufus Cornpone" by the New Frontier's Ivy Leaguers for his crude manner and Texas accent, deftly excluded by JFK from participation in policymaking, and openly detested by Robert Kennedy — a sentiment heartily reciprocated by LBJ in what Caro dubs "one of the great blood feuds in American political history." For a man who had dedicated his whole life to acquiring power and wielded immense amounts of it in the Senate, this was bitter indeed. Relegated to the sidelines, Johnson assumed a mask of deferential subordination that chafed miserably but never slipped to reveal the fury and frustration underneath. Caro implies that his vice presidential comeuppance tempered LBJ in ways that proved salutary after he was unexpectedly returned to power.

Made president by a shocking act of violence, Johnson knew he had to assure Americans of their government's continuity and stability. The iron self-control required of him as vice president served him well as he worked to persuade hostile Cabinet members and White House staff that they could best honor John F. Kennedy's legacy by remaining at his successor's side. And they did, even Bobby Kennedy, as Johnson committed himself to the passage of JFK's civil rights bill in his address to Congress on Nov. 27, 1963 — just five days after JFK was assassinated — and as he declared "unconditional war on poverty in America" in his first State of the Union message on Jan. 8, 1964.

Johnson had been urged to back off the civil rights bill by advisors who deemed it impossible to overcome the implacable opposition of the Senate's powerful Southern Democrats. "A President shouldn't waste his power on lost causes," one of them said. "Well, what the hell's the presidency for?" Johnson replied. In the past, ambition had always trumped his compassion for the poor and persecuted; now his superb political skills could be dedicated to something other than his personal advancement, and they were.

"The cliché says that power always corrupts," Caro writes, "but what is equally true is that power always reveals." The unswerving commitment to civil rights and the eradication of poverty revealed in "The Passage of Power" give it a different tone from its predecessors. Its tone of sympathy and admiration for a man who "not only had held the country steady during a difficult time but had set it on a new course, a course toward social justice." To do that, Johnson wrought astonishing changes within himself. His brutal arrogance, lachrymose self-pity, cruelty to subordinates, gloating boastfulness, penchant for secrecy and deception — qualities Caro has unsparingly depicted elsewhere and catalogs again in the last chapter — were suppressed as Johnson assumed command with an air of "disciplined calmness" that suggested he was fulfilled at last.

These qualities would resurface, we are warned in the foreboding final pages, which glance ahead to the nation's nightmarish entanglement in Southeast Asia. Vietnam was merely a cloud on the horizon during LBJ's first months as president, a critical time in American history that Caro judges "perhaps [Lyndon Johnson's] finest moment … not only masterful, but, in its way, heroic." With his habitual clear-eyed assessment of a very flawed human being warmed by appreciation for that unexpected heroism, "The Passage of Power" quite possibly will stand as Robert Caro's finest moment as well.

Smith is a contributing editor for the American Scholar and reviews books for The Times, the Chicago Tribune and the Washington Post.

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